The invention relates to an improved method for treating metal products. It applies in particular to heat treating of products obtained by rolling or continuous casting, such as steel slabs.
The term "treatment" when applied to metal products generally refers to sets of different operations usually comprising examination of the product with a flaw detector, elimination of the detected flaws, monitoring of said elimination, and possibly a final cleaning of the product.
The treatment proposed by the present invention relates to surface and subsurface ("subcutaneous") defects, i.e., defects disposed at depths not exceeding 10 mm from the surface of the product.
In the following description, reference is made in particular to continuous cast steel slabs. Obviously, the description is in no way limiting in character; the inventive method may equally be applied to numerous other metal products.
Steel works have always striven to produce products with minimum defects, so as to ensure finished products of high quality. However, to date it has not been possible to produce slabs which are complete free of defects. Consequently there is a need to detect and eliminate the kinds of defects which may be detrimental to the quality of finished products.
Treatment techniques currently employed are mostly empirical in nature, and their effectiveness tends to be dependent on the skill and experience of the operator.
One method in particular is known according to which the slab is cooled and a zone along the longitudinal axis on the top face of the slab is scarfed with a torch. This scarfed zone is generally not more than 5 mm deep and is 50 to 60 mm long. When the torch flame encounters a defect its shape and luminous intensity are altered momentarily; the operator must interpret these changes in order to assess the nature and degree of the defect.
In the case of certain very high quality steels, an additional two zones are sometimes scarfed, disposed near the longitudinal edges of the slab, parallel to the scarfed zone which was executed on the logitudinal axis.
In either case, the quality of the entire surface is assessed by extrapolating the results obtained by scarfing the zone(s) on the top face of the slab.
This method is very slow, and its empirical nature stands in the way of attainment of objective and reproducible results. Moreover, it basically only covers a small part of the surface of the product, and thus does not provide any real, practicable information concerning the rest of the surface.
There is also known a point scarfing apparatus, employed when gross defects are sought to be eliminated. A frame or carriage bearing a certain number of torches is displaced automatically along the slab to be treated, and the torches clean the surface by their action (scarfing, flame chipping, descaling, torch deseaming, etc.), at locations the coordinates of which are communicated to the control means of the said apparatus.
However, in order to use this apparatus, one must previously have located the defects which are to be eliminated. Furthermore, the apparatus does not allow the treated zone or the scarfing operation itself to be inspected; thus one suffers the hazards of operating blind.
A more recent method, applicable to products maintained at a temperature of 800.degree. to 1000.degree. C., consists of scarfing a zone of the type indicated supra, and transmitting the image of the flame of the torch to a remote location, with the aid of a television camera and monitor. The operator may thus observe the flame without being exposed to the heat emitted by the product at high temperature. The method gives the coordinates (the positions in the said zone) of the defects as indicated by the variations in shape and luminosity of the flame. The hot product is then conveyed to a treatment stand, where an automatically controlled (second) torch eliminates the defects at points the coordinates of which are communicated to the control means of the said automatic torch.
This method has the disadvantages inherent in the scarfing method discussed supra, namely slowness, empirical nature, lack of reliability and reproducibility of the observations, and small extent of the part of the surface observed. Furthermore, there is the inherent disadvantage of separation of the operations of detection and defect-elimination, as a result of which substantially more floor space and processing time are required, with attendant greater cost of the operation.
In addition, current treatment methods are essentially discontinuous, since in most cases they require that the product be transferred into an appropriate workshop (called a "treatment shop") which is separate from the casting room. This discontinuity disadvantage is the more serious as a greater proportion of the steel produced is continuously cast.